This invention relates generally to the field of hydrogen production and more specifically relates to methods of producing hydrogen by catalytic decomposition of water.
Concerns over global climate change are driving nations to reduce emissions of carbon dioxide to the atmosphere. Carbon dioxide is one compound believed to have a major impact on the earth""s thermal balance, thereby causing warming of our planet. Almost all of this carbon dioxide is produced in one way or another from fossil fuels such as coal, petroleum, and natural gas. There is a need to responsibly meet the 21st century energy needs in an environmentally benign manner.
In response to global climate change concerns, hydrogen is considered the fuel of choice for both power and transportation industry sectors. Its combustion product is water, not carbon dioxide. Hydrogen is also a valuable raw material for several major applications including refining and the manufacture of chemicals (e.g., ammonia). Hydrogen turbines are envisioned for electricity generation in the future and fuel cells are expected to power the transportation sector. These applications would significantly increase the demand for hydrogen.
Hydrogen is currently produced from fossil fuels such as natural gas by either steam reforming or partial oxidation. Steam reforming is a capital- and energy-intensive process. The process is thermally inefficient, and a portion of the feedstock must be burned to supply the energy to sustain the process, thereby generating some carbon dioxide. Partial oxidation is an exothermic reaction, but is again, very capital intensive. The hydrogen concentration in the product stream is lower than the steam reforming process and may not be suitable for many desired end products. To produce additional hydrogen from either process or to adjust the composition of the product gas, a water gas shift reaction is usually employed. This reaction further exacerbates the generation of carbon dioxide.
Separation of hydrogen from steam reformer or partial oxidation process product streams is another challenge. Pressure Swing adsorption or separation by membranes are some options. Current membrane systems, however, require a large pressure drop and gas recompression for downstream processes, are susceptible to chemical damage from trace sulfur compounds and aromatics, and have limited temperature and pressure tolerances. Polymeric membrane are low-temperature processes and, in general, separation by size-exclusion filtration does not yield industrially-significant hydrogen flux. Palladium-based membranes can separate hydrogen, but thin depositions of palladium on suitable porous substrates are not yet commercial. Hydrogen flux, is therefore, unacceptably low for palladium-based separation processes.
With concerns over global warming and the potential for hydrogen as the fuel of choice in the future, there is a need for more efficient and cost-effective means of producing large quantities of hydrogen without the attendant disadvantages of conventional technologies. In particular, a need exists to manufacture high-volume merchant hydrogen from low-cost feedstock, especially renewable resources, such as water, by an efficient, low-cost, and less energy intensive process. Hydrogen can be produced by the electrolysis of water, but the voltage required to split water molecules are excessive. Although food-grade hydrogen can be produced electrolytically, an electrically-driven process is not deemed to be commercially attractive for large scale applications.
Thin and dense ceramic membranes fabricated from mixed protonic and electronic conductors might provide a simple and efficient means for the production and separation of hydrogen from renewable resources. The preferred embodiment is a single step process that will split the water molecule into hydrogen and oxygen followed by separation of hydrogen via transport as proton through the membrane reactor wall thereby favoring the water splitting equilibrium towards the production of more hydrogen.
A method of producing hydrogen from water, in accordance with the present invention, comprises a method providing the removal of hydrogen from a gas stream, using mixed protonic/electronic conducting ceramic membranes.
The hydrogen producing method according to the present invention comprises the collection of hydrogen that is present in the gas stream as well as production of additional hydrogen by shifting equilibrium-limited reactions toward hydrogen products. In one exemplary embodiment of the present invention, a composite membrane is made from a proton conductor and a second phase material. The proton conductor preferably includes a lanthanide element, a Group VIA element, and a metal ion selected from the Group IA elements, Group IIA elements, and combinations of Group IA and Group IIA elements. More preferred proton conductors for use in the present invention include cerium, oxygen and a Group IA element such as barium or strontium. Still more preferred proton conductors also include yttrium. Preferred second phase materials include transition metals such as, but not limited to platinum, palladium, nickel, iron, cobalt, chromium, manganese, vanadium, silver, gold, copper, rhodium, ruthenium, niobium, zirconium, tantalum, or combinations thereof. Still more preferred second phase materials include nickel, palladium, niobium, or combinations thereof. A proton-conducting membrane is placed in an atmosphere that contains water and it generates hydrogen by catalyzing the splitting of water into hydrogen and oxygen and then separates the hydrogen from the oxygen. In preferred hydrogen-producing methods, separating the hydrogen from the oxygen further shifts the water splitting equilibrium toward the production of more hydrogen, which is collected by the proton-conducting membrane. The result is a plentiful supply of hydrogen at comparatively low temperatures.
It is one object of the present invention to provide a cost-effective means of producing hydrogen. To this end, a ceramic/metal composite is provided that catalyzes the splitting of water into hydrogen and oxygen. By decomposing water, which is a renewable resource, the cost of starting materials is reduced.
It is a further object of the present invention to provide large quantities of hydrogen. In the furtherance of this and other objectives, a method for producing large quantities of hydrogen is provided. In a preferred embodiment of the present invention, a mixed conductor composite membrane is provided. It is preferable that a proton conductor and a second phase material be provided. In a preferred embodiment in accordance with the present invention the proton conductor includes a lanthanide element, a Group VIA element, and a metal ion formed from a Group IA element, a Group IIA element, or combinations of these elements. More preferred proton conductors include cerium, oxygen and a metal ion of a Group IIA element such as barium, strontium, or combinations of these elements. Yttrium is included in more preferred proton conductors. More preferred second phase materials include nickel, palladium, niobium, or combinations of these materials.
Additional hydrogen may be produced, in accordance with the present invention, by shifting equilibrium-limited reactions toward the production of hydrogen. A preferred embodiment provides the splitting of hydrogen and oxygen coupled with the sequestration of the hydrogen in a single step. In a preferred embodiment in accordance with the present invention, hydrogen is collected by the proton-conducting membrane.
It is yet another object of the present invention to provide a method of producing hydrogen without producing undesirable byproducts. To this end, a mixed conductor membrane is provided.
Still another objective of the present invention is to provide a method of decomposing water into hydrogen and oxygen at temperatures below about 2,000xc2x0 C. In the furtherance of this and other objectives, a ceramic/metal composite is provided to separate hydrogen from gas mixtures. This allows the process to be carried out at temperatures of about 900xc2x0 C.
Further objects, features and advantages of the invention will be apparent from the following detailed description taken in conjunction with the accompanying drawings.